Regions
Northwest Coastal.
Extra caution is needed in areas in the northern part of the region where buried persistent weaknesses remain active.
Confidence
Moderate - Due to the number of field observations
Weather Forecast
Light flurries and moderate southerly winds are forecast for Sunday and Monday. A mix and sun and cloud and light winds are expected on Tuesday. Freezing levels should sit at about 1300m for the forecast period.
Avalanche Summary
On Thursday, there were reports of natural and explosive-triggered cornice falls, primarily from areas north of Stewart and near Ningunsaw Pass. Many of these triggered persistent slabs with the largest results (size 3-3.5) occurring on NE-NW aspects. Observations from the southern part of the region were limited, but in the past few days there were a few reports of natural and human triggered storm slabs to size 2 just below ridgecrest in north facing alpine terrain.
Snowpack Summary
Easterly winds have redistributed the surface snow forming fresh wind slabs in exposed terrain. On sheltered and shady slopes there is 20-40 cm of settled dry (and faceting) powder, possibly capped by surface hoar. On south aspects, the recent storm snow may be sitting on a sun crust. Lower elevations and south aspects are going through daily melt-freeze cycles. A layer of surface hoar or melt-freeze crust buried on March 3 is down 70-100cm and has been recently reactive in the areas north of Stewart. An older crust/facet layer buried in early February can now be found down over a meter but this layer has been dormant for several weeks.
Problems
Cornices
Cornice Fall is the release of an overhanging mass of snow that forms as the wind moves snow over a sharp terrain feature, such as a ridge, and deposits snow on the downwind (leeward) side. Cornices range in size from small wind drifts of soft snow to large overhangs of hard snow that are 30 feet (10 meters) or taller. They can break off the terrain suddenly and pull back onto the ridge top and catch people by surprise even on the flat ground above the slope. Even small cornices can have enough mass to be destructive and deadly. Cornice Fall can entrain loose surface snow or trigger slab avalanches.
Wind Slabs
Wind Slab avalanches are the release of a cohesive layer of snow (a slab) formed by the wind. Wind typically transports snow from the upwind sides of terrain features and deposits snow on the downwind side. Wind slabs are often smooth and rounded and sometimes sound hollow, and can range from soft to hard. Wind slabs that form over a persistent weak layer (surface hoar, depth hoar, or near-surface facets) may be termed Persistent Slabs or may develop into Persistent Slabs.
Persistent Slabs
Persistent Slab avalanches are the release of a cohesive layer of snow (a slab) in the middle to upper snowpack, when the bond to an underlying persistent weak layer breaks. Persistent layers include: surface hoar, depth hoar, near-surface facets, or faceted snow. Persistent weak layers can continue to produce avalanches for days, weeks or even months, making them especially dangerous and tricky. As additional snow and wind events build a thicker slab on top of the persistent weak layer, this avalanche problem may develop into a Deep Persistent Slab.